The invention relates to vibroseis operations and, more specifically, to the processing and analysis of signals transmitted through subsurfaces, either directly, or after various reflections on various substrata of such a subsurface.
It is frequently sought to eliminate distortions or correlation noise of such signals, which appear at the stage where logged signals are finally processed.
These phenomena mainly result from the undesirable appearance of harmonics of the emitted signal, harmonics which it is therefore desired to eliminate on receiving the signals.
Typically, the signal is emitted by several vibrators and is in the form of a frequency sweep. The sweep is typically repetitive and often linear. A linear and repetitive signal is thus known as a xe2x80x9cslip-sweepxe2x80x9d signal.
A slip-sweep seismic acquisition method described by H. J. Rozemond during the 66th SEG meeting in 1996, (Slip-Sweep acquisition) provided for the separation of vibroseis signals emitted by various sources and overlapping in terms of time.
The seperation envisaged is only perfect if one of the two following conditions is met:
the signal has no distortion;
the time difference between two successive emissions is long enough that the correlation noise associated with a source does not interfere with the signal associated with the other sources.
In practice, no distortion-free vibroseis source is known, and, furthermore, the need to optimize the productivity of the seismic acquisition leads to searching for time differences between vibrations which are as short as possible.
Under these conditions, the recordings obtained have a signal-to-noise ratio which is worse than that which would be obtained by using sources without a time overlap.
Patent GB 2 348 003 describes a method to reduce the correlation noise. This method is applicable to sets of seismograms which beforehand have been processed and grouped into mirror points (such that the reflections from the same point in space are at the same point in time or in depth). These seismograms are then decomposed into narrow frequency bands in which statistical discrimination of the signal and of the noise is carried out.
Other methods have been proposed to improve vibroseis productivity.
For example, it has been proposed to encode the phase of signals emitted simultaneously by n groups of vibrators.
It has been shown that if n successive recordings are carried out with suitably adjusted phases, it is possible to separate the signals emitted by the n groups of vibrators. However, the separation is complete only for the fundamental part of the signal and not for its harmonics.
Another possibility is to emit simultaneously in separate frequency bands. The signals generated by the various sources are mutually orthogonal and consequently may be separated from each other. However, the orthogonality is only completely applicable to the fundamentals, the presence of harmonics resulting in excess noise.
Reduction in the correlation noise is therefore one of the keys to increasing vibroseis productivity, and the techniques proposed to date have been shown to be unsatisfactory.
The main aim of the present invention is a method of improved efficiency for eliminating harmonics in a vibroseis signal.
The aim of the present invention is thus especially to make it possible to reduce the correlation noise in individual seismograms, for example before any processing, by using the time/frequency transform to separate the signal from the correlation noise.
These aims are achieved according to the invention using a vibroseis analysis method in which frequency-sweep signals are emitted into a subsurface, the signals reflected on the substrata of such a subsurface are logged and the logged signals are processed, a method in which the harmonics of the fundamental signal initially emitted are eliminated from the logged signals, by applying the steps consisting in:
a) providing a time/frequency plot, showing the respective contributions of the fundamental and of the harmonics in the logged signal,
b) providing a time/frequency plot also showing these contributions of the fundamental and of the harmonics in the logged signal, this plot having been stretched in the direction of the frequency axis such that the fundamental of this plot is over the location of a harmonic chosen from the plot;
c) adapting the power amplitude of this stretched plot to make this amplitude correspond to that of the said chosen harmonic of the plot;
d) subtracting these two plots one from the other such that the said chosen harmonic is eliminated, by subtraction with the fundamental of the stretched plot.